I'm starting to notice a "proficiency bonus +n" in some monsters' stat blocks that I don't see in the MM. Doppelganger, for example, has a +2. What are the circumstances where that would be applied?
The proficiency bonus is used to calculate attack bonuses, saving throws, and skill proficiencies.
You can see that the Doppleganger's Insight check is +3. That comes from wisdom (+1) and Proficiency Bonus (+2).
Generally speaking, you shouldn't need to do anything with it directly, however, if you were to have the Doppleganger do something unlisted that it is proficient in, you could add it to its roll.
I'm starting to notice a "proficiency bonus +n" in some monsters' stat blocks that I don't see in the MM. Doppelganger, for example, has a +2. What are the circumstances where that would be applied?
The proficiency bonus is used to calculate attack bonuses, saving throws, and skill proficiencies.
You can see that the Doppleganger's Insight check is +3. That comes from wisdom (+1) and Proficiency Bonus (+2).
Generally speaking, you shouldn't need to do anything with it directly, however, if you were to have the Doppleganger do something unlisted that it is proficient in, you could add it to its roll.
This was likely added due to Tasha and the sidekicks and companions therein.